1. Field of the Invention:
This invention relates in general to oil well drilling equipment. In particular, the invention relates to lifting heads for handling drill collars.
2. Description of the Prior Art:
Drill collars are heavy, thick-walled tubular members, located in a drill stem between the drill pipe and the bit. The purpose of the drill collar is to place weight on the bit in order to improve the bit's performance. Unlike sections of drill pipe, drill collars usually do not have upset sections at each end. Lifting heads for drill collars must, therefore, grip the drill collar surface, or a sub must be added to the drill collar.
Some drill collars have spiral cuts which cover practically the entire length of the drill collar. These spiral cuts prevent, or at least greatly reduce the likelihood of, differential wall sticking. If a lifting head engages a drill collar in an area which has a spiral cut, the gripping inserts on the lifting head may not contact the surface of the collar.